On Thu, May 02 2019, Andreas Gruenbacher wrote: > On Thu, 2 May 2019 at 05:57, NeilBrown <neilb@xxxxxxxx> wrote: >> On Wed, May 01 2019, Amir Goldstein wrote: >> > On Wed, May 1, 2019 at 10:03 PM NeilBrown <neilb@xxxxxxxx> wrote: >> >> On Tue, Dec 06 2016, J. Bruce Fields wrote: >> >> > On Tue, Dec 06, 2016 at 02:18:31PM +0100, Andreas Gruenbacher wrote: >> >> >> On Tue, Dec 6, 2016 at 11:08 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: >> >> >> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas Grünbacher >> >> >> > <andreas.gruenbacher@xxxxxxxxx> wrote: >> >> >> >> 2016-12-06 0:19 GMT+01:00 Andreas Grünbacher <andreas.gruenbacher@xxxxxxxxx>: >> >> >> > >> >> >> >>> It's not hard to come up with a heuristic that determines if a >> >> >> >>> system.nfs4_acl value is equivalent to a file mode, and to ignore the >> >> >> >>> attribute in that case. (The file mode is transmitted in its own >> >> >> >>> attribute already, so actually converting .) That way, overlayfs could >> >> >> >>> still fail copying up files that have an actual ACL. It's still an >> >> >> >>> ugly hack ... >> >> >> >> >> >> >> >> Actually, that kind of heuristic would make sense in the NFS client >> >> >> >> which could then hide the "system.nfs4_acl" attribute. > > I still think the nfs client could make this problem mostly go away by > not exposing "system.nfs4_acl" xattrs when the acl is equivalent to > the file mode. Maybe ... but this feels a bit like "sweeping it under the carpet". What happens if some file on the lower layer does have a more complex ACL? Do we just fail any attempt to modify that object? Doesn't that violate the law of least surprise? Maybe if the lower-layer has an i_op->permission method, then overlayfs should *always* call that for permission checking - unless a chmod/chown/etc has happened on the file. That way, we wouldn't need to copy-up the ACL, but would still get correct ACL testing. Thanks, NeilBrown > The richacl patches contain a workable abgorithm for > that. The problem would remain for files that have an actual NFS4 ACL, > which just cannot be mapped to a file mode or to POSIX ACLs in the > general case, as well as for files that have a POSIX ACL. Mapping NFS4 > ACL that used to be a POSIX ACL back to POSIX ACLs could be achieved > in many cases as well, but the code would be quite messy. A better way > seems to be to using a filesystem that doesn't support POSIX ACLs in > the first place. Unfortunately, xfs doesn't allow turning off POSIX > ACLs, for example. > > Andreas > >> >> >> > Even simpler would be if knfsd didn't send the attribute if not >> >> >> > necessary. Looks like there's code actively creating the nfs4_acl on >> >> >> > the wire even if the filesystem had none: >> >> >> > >> >> >> > pacl = get_acl(inode, ACL_TYPE_ACCESS); >> >> >> > if (!pacl) >> >> >> > pacl = posix_acl_from_mode(inode->i_mode, GFP_KERNEL); >> >> >> > >> >> >> > What's the point? >> >> >> >> >> >> That's how the protocol is specified. >> >> > >> >> > Yep, even if we could make that change to nfsd it wouldn't help the >> >> > client with the large number of other servers that are out there >> >> > (including older knfsd's). >> >> > >> >> > --b. >> >> > >> >> >> (I'm not saying that that's very helpful.) >> >> >> >> >> >> Andreas >> >> >> >> Hi everyone..... >> >> I have a customer facing this problem, and so stumbled onto the email >> >> thread. >> >> Unfortunately it didn't resolve anything. Maybe I can help kick things >> >> along??? >> >> >> >> The core problem here is that NFSv4 and ext4 use different and largely >> >> incompatible ACL implementations. There is no way to accurately >> >> translate from one to the other in general (common specific examples >> >> can be converted). >> >> >> >> This means that either: >> >> 1/ overlayfs cannot use ext4 for upper and NFS for lower (or vice >> >> versa) or >> >> 2/ overlayfs need to accept that sometimes it cannot copy ACLs, and >> >> that is OK. >> >> >> >> Silently not copying the ACLs is probably not a good idea as it might >> >> result in inappropriate permissions being given away. >> > >> > For example? permissions given away to do what? >> > Note that ovl_permission() only check permissions of *mounter* >> > to read the lower NFS file and ovl_open()/ovl_read_iter() access >> > the lower file with *mounter* credentials. >> > >> > I might be wrong, but seems to me that once admin mounted >> > overlayfs with lower NFS, NFS ACLs are not being enforced at all >> > even before copy up. >> >> I guess it is just as well that copy-up fails then - if the lower-level >> permission check is being ignored. >> >> > >> >> So if the >> >> sysadmin wants this (and some clearly do), they need a way to >> >> explicitly say "I accept the risk". If only standard Unix permissions >> >> are used, there is no risk, so this seems reasonable. >> >> >> >> So I would like to propose a new option for overlayfs >> >> nocopyupacl: when overlayfs is copying a file (or directory etc) >> >> from the lower filesystem to the upper filesystem, it does not >> >> copy extended attributes with the "system." prefix. These are >> >> used for storing ACL information and this is sometimes not >> >> compatible between different filesystem types (e.g. ext4 and >> >> NFSv4). Standard Unix ownership permission flags (rwx) *are* >> >> copied so this option does not risk giving away inappropriate >> >> permissions unless the lowerfs uses unusual ACLs. >> >> >> >> >> > >> > I am wondering if it would make more sense for nfs to register a >> > security_inode_copy_up_xattr() hook. >> > That is the mechanism that prevents copying up other security.* >> > xattrs? >> >> No, I don't think that would make sense. >> Support some day support for nfs4 acls were added to ext4 (not a totally >> ridiculous suggestion). We would then want NFS to allow it's ACLs to be >> copied up. >> >> Thanks, >> NeilBrown >> >> >> > >> > Thanks, >> > Amir.
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